This invention relates to a method for producing uniform and consistent slurries of a communited solid friable material and a liquid material. More particularly, this invention relates to a method for operating a rod mill to provide a uniform and consistent slurry having a high solids content, but which is still capable of being pumped and transported in relatively stable form.
Rod mills have been in use in industry for many years. Their function of size reduction of various solid materials is well known and thoroughly evaluated. As stated in Kirk-Othmer, Encyclopedia of Chemical Technology, (Third Ed.), Vol. 21, pages 154-161rod mills are one form of tumbling mills in which the grinding medium is a number of rods having one dimension several times the other and which are characterized by a cylinder containing the grinding medium rotating about a horizontal, or nearly horizontal, axis. The comminuting action takes place by the material being caught between grinding medium elements (rods) and the cylinder wall, by rolling-action encounters between the medium and the material and by impacts from the media. In general, rod mills have unique design characteristics inherent in the type of discharge means, i.e., overflow, endperipheral or centerperipheral. For each discharge type several characteristics will bear on the selection of one type of rod mill or the other, including the type of process, either wet or dry; the maximum reduction ratios; typical product size; and recommended rotating speed.
Size reduction and particularly rod mills are used in the chemical process industries mainly in the preparation of mineral feed stocks, e.g., limestone plus lime, phosphate rock, coal (as a chemical feedstock), silica plus feldspars, etc. With impure minerals, the size-reduction step may be incorporated into a mineral-processing plant to upgrade the ore to meet user specifications. Lesser but still important applications exist in coal pulverization for fuel, in the preparation of suitably sized products for sale, or in the preparation of gasification slurry feedstocks. Typical examples of size reduction for ores and minerals are the taconite (low grade iron ore) industry, in which a large-scale minerals processing plant utilized staged comminution with intermediate size separation and beneficiation to upgrade ores containing 20-40% Fe to a blast furnace feed of 64% Fe. In industrial nonmetallic minerals, such as the processing of limestone, the mineral is reduced from run-of-mine stone to desired sizes, primarily by open circuit jaw crushers, gyratories, or impact breakers, and then further with open or closed circuit cone crushers, hammer mills, or cage mills and occasionally roll crushers or rod mills. Fine grinding or pulverizing is carried out in hammer mill, cage mill, roller mills, and rod, ball, tube, pebble and vibrating mills. Other minerals of importance include phosphate rock for fertilizers; silica and feldspars for glass, ceramic, and refractories; and sylvinite (.gamma.NaCl.KCl) for the preparation of KCl, principally for fertilizers. The materials required for portland cement manufacture are lime, alumina, and silica obtained from limestone, shale, clay, or cement rock. Run-of-mine stone is first crushed in jaw crushers, gyratories, impact crushers, or toothed rolls, then blended in appropriate proportions and ground to 75-90% passing 75.mu.in wet or dry circuits. Modern plants prefer dry grinding in order to minimize fuel requirements for drying before calcination. Grinding is carried out, frequently in a single stage, in roller mills, ball mills, tube mills (long ball mills), and compartmented ball mills (2-3 compartments charged with different ball sizes). The material is dried in a separate dryer or in a mill-classifier closed circuit, employing waste heat from the cement kiln. Rod mills can also be used in this service. Pulverized coal is principally used in mainly bituminous form for combustion for energy and carbonization for coke, tar, coal, chemicals, and gas production.
In view of the important processes and industries requiring size reduction and using rod mills therefor, a method for operating a rod mill which provides for more precise control of the product and a more uniform product is not only desirable, but also necessary from a quality and competition standpoint. Therefore, one of the objects of this invention is to provide a method for operating a rod mill so that control of the mill is more precise and so that a uniform product slurry is obtained. These and other objects, which will be clear from the specification and which provide satisfaction in use, are provided by the present invention.